U.S. patent application number 12/943718 was filed with the patent office on 2012-05-10 for optical printed circuit board and a method of manufacturing an optical printed circuit board.
This patent application is currently assigned to Xyratex Technology Limited. Invention is credited to Richard C.A. PITWON.
Application Number | 20120114280 12/943718 |
Document ID | / |
Family ID | 46019699 |
Filed Date | 2012-05-10 |
United States Patent
Application |
20120114280 |
Kind Code |
A1 |
PITWON; Richard C.A. |
May 10, 2012 |
OPTICAL PRINTED CIRCUIT BOARD AND A METHOD OF MANUFACTURING AN
OPTICAL PRINTED CIRCUIT BOARD
Abstract
A method of manufacturing an optical printed circuit board and
an optical printed circuit board. The method includes providing a
support layer having one or more optical waveguides formed on the
support layer, the waveguides having exposed interfaces. A film is
provided on one or more of the exposed interfaces, wherein the film
has a smoother outer surface than the waveguide interface.
Inventors: |
PITWON; Richard C.A.;
(Fareham, GB) |
Assignee: |
Xyratex Technology Limited
Havant
GB
|
Family ID: |
46019699 |
Appl. No.: |
12/943718 |
Filed: |
November 10, 2010 |
Current U.S.
Class: |
385/14 ;
29/829 |
Current CPC
Class: |
H05K 1/0274 20130101;
G02B 6/4249 20130101; G02B 6/4214 20130101; G02B 6/4292 20130101;
G02B 6/4212 20130101; Y10T 29/49124 20150115 |
Class at
Publication: |
385/14 ;
29/829 |
International
Class: |
G02B 6/12 20060101
G02B006/12; H05K 3/00 20060101 H05K003/00 |
Claims
1. A method of manufacturing an optical printed circuit board, the
method comprising: providing a support layer having one or more
optical waveguides formed on the support layer, the optical
waveguides having exposed interfaces; and, providing a film on one
or more of the exposed interfaces, wherein the film has a smoother
outer surface than the waveguide interface.
2. A method according to claim 1, wherein the innermost surface of
the film conforms to the roughness profile of the waveguide
interface.
3. A method according to claim 1, wherein the film has an optical
index which matches the optical index of the waveguide to within
.+-.10% of the difference between the refractive index of the
waveguide core and that of the external environment.
4. A method according to claim 1, wherein the outer surface of the
film is at least one order of magnitude smoother than the waveguide
interface.
5. A method according to claim 1, wherein the film is preformed
before being applied to the exposed interface and the film has a
partially viscous inner surface portion that conforms to the
roughness profile of the waveguide interface.
6. A method according to claim 1, wherein the film is adhesively
applied to the exposed interfaces.
7. A method according to claim 6, wherein the film prior to being
applied to the interfaces has an adhesive backing.
8. A method according to claim 1, wherein there is a plurality of
waveguides having coplanar interfaces, the method comprising
applying a single piece of film over the plural interfaces.
9. A method according to claim 1, wherein providing the film
comprises: coating the waveguide interface with a liquid material;
applying the surface of a plane member to the liquid material, the
plane member being at least partially transparent and smoother than
the waveguide interface; applying light to the liquid material
through the at least partially transparent plane member to cure the
material to form said film; and, removing the plane member.
10. A method according to claim 9, wherein the plane member is
coupled to a shield member arranged to shield a portion of the
optical printed circuit board from being contaminated by said
liquid material.
11. A method according to claim 1, comprising attaching a holding
frame to the optical printed circuit board, the holding frame being
constructed and arranged to help in holding the film in place
against the waveguide interface or interfaces.
12. A method according to claim 11, wherein the holding frame
applies a tension to the film to keep it taut.
13. A method according to claim 11, wherein the holding frame is
constructed and arranged to provide at least one connector
receptacle and/or a lens and/or another light coupling device for
free-space coupling light to and/or from at least one
waveguide.
14. An optical printed circuit board comprising: a support layer;
on the support layer, one or more optical waveguides having exposed
interfaces; a film provided on one or more of the exposed
interfaces, wherein the film has a smoother outer surface than the
waveguide interface.
15. An optical printed circuit board according to claim 14, wherein
the innermost surface of the film conforms to the roughness profile
of the waveguide interface
16. An optical printed circuit board according to claim 14, wherein
the film has an optical index which matches the optical index of
the waveguide to within .+-.10% of the difference between the
refractive index of the waveguide core and that of the external
environment.
17. An optical printed circuit board according to claim 14, wherein
the outer surface of the film is at least one order of magnitude
smoother than the waveguide interface.
18. An optical printed circuit board according to claim 14, the
film has a partially viscous inner surface portion that conforms to
the roughness profile of the waveguide interface.
19. An optical printed circuit board according to claim 14, wherein
the film is adhesively applied to the exposed interfaces.
20. An optical printed circuit board according to claim 14, wherein
there are a plurality of waveguides having coplanar interfaces, a
single piece of film is applied over the plural interfaces.
21. An optical printed circuit board according to claim 14,
comprising a holding frame attached to the optical printed circuit
board, the holding frame being constructed and arranged to help in
holding the film in place against the waveguide interface or
interfaces.
22. An optical printed circuit board according to claim 21, wherein
the holding frame also stretches the film.
23. An optical printed circuit board according to claim 21, wherein
the holding frame is constructed and arranged to provide at least
one connector receptacle and/or a lens and/or another light
coupling device for coupling light to and/or from at least one
waveguide.
Description
[0001] The present invention relates to an optical printed circuit
board and a method of manufacturing an optical printed circuit
board.
[0002] Optical printed circuit board technologies have a wide range
of applications which take advantage of their ability to support
high bandwidths and other known benefits. For example, it is known
to use optical PCBs in aerospace vehicles to convey sensor
information to primary processing nodes, or as optical links in
large passenger planes to convey sensor and multimedia information
across the plane (e.g. video on demand for passengers). They can
also be used in automotive applications for the distribution of
sensor and media information in cars. They can be used in high
performance computing to accommodate huge bandwidth densities
around processor nodes, or in a telecommunications setting to
accommodate the huge bandwidth densities for embedded optical
channels at network nodes. It is also contemplated that optical
printed circuit boards may be advantageous in data storage
technologies, for example for signal communications in the back
planes of disk drive storage enclosures.
[0003] Optical printed circuit boards (PCBs) have optical
waveguides that are used for the transmission of light signals
between components, as well as or instead of conventional copper
conductors. Typically, an optical PCB consists of a base or support
layer. In areas of the optical PCB where optical waveguides are
required, a lower optical cladding layer is provided usually of
uniform thickness. On top of this, a layer of optical core material
is laid down. The optical core material has a higher refractive
index than the cladding layer and will eventually form one or more
optical waveguides on the optical PCB.
[0004] In a known process used for making optical PCBs, the core
layer is laid down in liquid form, e.g. as a curable liquid
polymer. A photolithographic mask having a pattern corresponding to
the desired shape of the waveguides is arranged over the liquid
polymer and the entire resultant structure is then irradiated with
electromagnetic radiation of suitable wavelength. Thus, in regions
of the mask which are open, the liquid polymer is cured. In other
regions, the polymer remains liquid. The mask is removed and the
remaining liquid polymer can be washed away with an appropriate
solvent known as a "developer" leaving the desired pattern of
optical waveguides. Alternatively a dry film optical polymer can be
used instead of a liquid polymer.
[0005] The remaining core material is typically arranged in
patterns of channels which are arranged in some manner so as to be
able to couple optical signals between components on the optical
PCB when the components are arranged thereon. Last, an upper
cladding layer is laid down, so that the channels of core material
are completely surrounded by cladding material, and therefore are
able to function as optical waveguides. FIG. 1 shows a schematic
representation of such a conventional optical PCB and FIG. 2 shows
a sectional view of the same optical PCB.
[0006] The PCB 2 comprises a base or support layer 4 upon which is
arranged a lower cladding layer 6. A plurality of optical
waveguides 8 are arranged on the lower cladding layer 6 and have
arranged around them an upper cladding layer 10. Thus, the optical
waveguides are entirely surrounded by cladding material of the
upper cladding layer 10 and the lower cladding layer 6. Typically,
the height H of the optical waveguides will be of the order 50 to
70 .mu.m. The width W of each of the optical waveguides is
typically of the order 50 to 100 .mu.m. The waveguides can be
fabricated to very high accuracy, typically of the order of <1
.mu.m. High accuracy is an important requirement of any optical
waveguide structure.
[0007] In order to couple optical signals into the waveguides 8,
optical components such as transmitters, receivers, 45.degree.
mirrors and other optical waveguides, the components must be
aligned with respect to the optical waveguide input interfaces or
facets 12 with a high alignment tolerance. It is also important to
achieve a smooth and normally planar interface at the facets 12 to
avoid scattering of light entering or exiting the waveguide 8.
[0008] For example, FIG. 3 shows a magnified view of a facet 12
showing a typical roughness profile of a conventionally
manufactured optical PCB 2. FIG. 4 shows the effect this roughness
produces on light 8 propagating in the waveguide 8 and exiting the
facet 12. The roughness of the surface 14 causes scattering of
light 24 as the light exits the waveguide 8, causing in turn the
attenuation of the optical signal 22. It should be noted that while
FIG. 3 shows an output facet 12, the problem of scattering and
attenuation of light affects input facets 12 in a similar way. This
loss of light is a major problem to be addressed in the manufacture
of optical PCBs 2.
[0009] Currently, the most effective method of reducing this loss
is manual polishing. However, this method is not very effective.
Even using this method, the roughness of the facets 12 can cause
over 50% of the total optical link loss in the waveguide. Another
known technique for reducing attenuation of light due to facet
roughness 14 is to use laser ablation of the waveguide facets 12.
However, both these methods are unreliable and unrepeatable. In
particular, any damage incurred by these methods will be difficult
to correct and therefore using these techniques is likely to
significantly impact yield in manufacturing optical PCB technology.
These methods are also difficult and costly to apply. What is
needed is a repeatable, reliable and low cost method of preparing
the exposed input and output facets of the waveguide to reduce
scattering at the interface.
[0010] In instances where non-free space interfaces are used, i.e.
where a component couples directly to a waveguide, it is known to
apply an index matching oil or gel onto the interface between the
waveguide and component to reduce coupling losses. However, this
method is unsuitable for an application requiring repeatable
engagement and disengagement of the component to the waveguide as
the index matching material would flow out during disengagement and
attract and hold contaminants (e.g. dust particles), contaminate
other components or simply dry up and cease to be effective when
next required.
[0011] According to a first aspect of the present invention, there
is provided a method of manufacturing an optical printed circuit
board, the method comprising:
[0012] providing a support layer having one or more optical
waveguides formed on the support layer, the waveguides having
exposed interfaces; and,
[0013] providing a film on one or more of the exposed interfaces,
wherein the film has a smoother outer surface than the waveguide
interface.
[0014] The invention provides a low-cost, repeatable method of
creating smooth surfaces on cheaply cut waveguides as part of an
optical PCB fabrication process. This counteracts optical
scattering loss due the roughness of exposed input and output
interfaces (also known as facets) of a PCB embedded optical
waveguide. Optical scattering at waveguide facets is currently a
major problem in embedded waveguide technology and is often a
critical failure mode, as scattering can give rise to over half the
entire link loss on an optical channel.
[0015] Preferably, the innermost surface of the film conforms to
the roughness profile of the waveguide interface. This minimises
scattering at the interface between the film and the waveguide.
[0016] Preferably, the film has an optical index which matches the
optical index of the waveguide to within .+-.10% of the difference
between the waveguide core refractive index and the external
environment. The external environment in most cases will be free
space (e.g. air).
[0017] This helps reduce distortion of the light as it passes from
the waveguide to the film. Optical scattering due to surface
roughness of the waveguide end facet increases with the difference
in refractive index between the waveguide core and the external
free space medium outside the waveguide core (typically air). By
providing an intermediary refractive index step, this scattering
effect is dampened and in the ideal situation whereby the thin film
has the same or similar refractive index as the waveguide core, the
scattering loss is minimised at the interface between the waveguide
core and the film. Being within 10% of said difference is
particularly preferred. However, the invention would be
advantageous compared with a raw waveguide end facet for percentage
differences as large as 30% and even larger depending on the
trade-off of loss due to pure roughness, absorption of the film
itself (which should be small as the film will be thin) and
scattering due to the mismatch in refractive index.
[0018] Preferably the film is thin. For example a preferred film
can be between 20 .mu.m to 100 .mu.m thick. Nonetheless, a film as
thick as 500 .mu.m could be useful depending on said trade-off.
[0019] Preferably, the outer surface of the film is at least one
order of magnitude smoother than the waveguide interface. This
significantly reduces the loss due to scattering, by as much as 3
dB in tests.
[0020] Surface roughness is typically expressed in terms of the
following parameters denoting the vertical deviations of the
surface profile from the mean line:--
[0021] Ra=arithmetic average of absolute values
[0022] Rv=maximum valley depth
[0023] Rp=maximum peak height
[0024] Rq=root mean squared
[0025] Rq is most typically used to quantify surface roughness.
Typical Rq values for cut waveguides range from 600 nm to 200 nm
depending on the method used to cut them.
Certain more expensive cutting tools such as diamond saws can give
much smoother finishes such as Rq values as low as 60 nm. However
in many optical printed circuit board applications, such tools
cannot be used, for instance when creating waveguide interface
points within the board requiring very small slots to be built.
[0026] Preferably the outer surface is two, three, four or more
orders of magnitude smoother. This greatly reduces scattering as
the light exits or enters the waveguide.
[0027] This can be done by making the inner surface of the film
soft and to some degree viscous so this reduces to a minimum any
scattering at the interface between the film and the waveguide
interface.
[0028] The invention is particularly useful in achieving a smooth
waveguide interface in so-called free-space applications, where no
component couples directly to the waveguide. A typical example
would be a free space lens coupling arrangement whereby a lens
system is arranged at a fixed distance away from the output facet
of the waveguide. This fixed distance would be required to allow
light rays entering or exiting the waveguide to converge into or
diverge out of the waveguide according to the focal length
requirements of the lens system.
[0029] Preferably, the film is preformed before being applied to
the exposed interface and the film has a partially viscous inner
surface portion that conforms to the roughness profile of the
waveguide interface. This allows the profile of the inner surface
to "flow" and fill out the uneven profile of the waveguide
facet.
[0030] Preferably, the film is adhesively applied to the exposed
interfaces. This provides a convenient way of applying the film to
the facet and securing it into position. The adhesive can also help
conform the surface of the film to the roughness profile of the
waveguide interface.
[0031] Preferably, the film prior to being applied to the
interfaces has an adhesive backing. This provides a convenient way
of applying the film. By selecting a suitable adhesive, the film
can be easily removed and reapplied thus providing a means of
significantly increasing optical PCB fabrication yield.
[0032] Preferably, there is a plurality of waveguides having
coplanar interfaces, the method comprising applying a single piece
of film over the plural interfaces.
[0033] This simplifies manufacture by allowing multiple waveguide
interfaces to be made smooth by the application of a single piece
of film in one operation. This is particularly useful where the
interfaces are in a localised region on the PCB, e.g. within a few
square centimetres. In a typical example, the film may be about 6
mm wide and 2 mm high and the waveguides may be stacked vertically
as well as horizontally.
[0034] In another embodiment, providing the film comprises:
[0035] coating the waveguide interface with a liquid material;
[0036] applying the surface of a plane member to the liquid
material, the plane member being at least partially transparent and
smoother than the waveguide interface;
[0037] applying light to the liquid material through the at least
partially transparent plane member to cure the material to form
said film; and,
[0038] removing the plane member.
[0039] This method allows a different way of providing a smooth
film on the surface of the waveguide. Rather than applying a
preformed film to the facet, a film can be provided by coating the
facet with a liquid material, applying a plane member to create the
smooth outer surface, and curing the liquid to make it into a solid
film. The plane member can then be removed.
[0040] The plane member may be coupled to a shield member arranged
to shield a portion of the optical printed circuit board from being
contaminated by said liquid material. For example, the edge of the
optical circuit board may have alignment features or other portions
that it is important to keep clear of contamination. The shield
member may be constructed and arranged to hold the plane member and
to shield the relevant portions of the optical PCB to prevent the
liquid material coming into contact with the portions of PCB when
the plane member is applied against the liquid material.
[0041] In an embodiment, the method comprises attaching a holding
frame to the optical printed circuit board, the holding frame being
constructed and arranged to help in holding the film in place
against the waveguide interface or interfaces.
[0042] The frame helps hold the film in place and prevent
accidental removal and damage to the film. For example, the frame
can be attached to the PCB by any suitable means, such as a clip,
fastener or adhesive. Preferably the frame engages the film around
the periphery of the film to allow clear access to the waveguide
interfaces towards the central portion of the film.
[0043] Preferably, the holding frame also stretches the film.
[0044] The frame can therefore help prevent wrinkles or other
mechanical deformations of the film when in the operational
environment.
[0045] Preferably, the holding frame is constructed and arranged to
provide at least one connector receptacle and/or a lens holder
and/or another light coupling device for free-space coupling light
to and/or from at least one waveguide.
[0046] When free-space coupling light to or from a waveguide, it is
of critical importance to achieve the correct separation between
the light coupling device and the waveguide facet. Due to the
fabrication tolerances inherent to PCBs it is difficult to ensure
that this precise distance is maintained. The frame allows a light
coupling device to be mounted and the film has a defined and very
smooth outer surface, which allow a far better mechanical
registration point in the z-axis (along the axis of the waveguides
at the interface) compared to prior art techniques.
[0047] According to a second aspect of the present invention, there
is provided an optical printed circuit board comprising:
[0048] a support layer;
[0049] on the support layer, one or more optical waveguides having
exposed interfaces;
[0050] a film provided on one or more of the exposed interfaces,
wherein the film has a smoother outer surface than the waveguide
interface.
[0051] Examples of the present invention will now be described in
detail with reference to the accompanying drawings, in which:
[0052] FIG. 1 shows a conventional optical PCB;
[0053] FIG. 2 shows schematic section through the optical PCB of
FIG. 1;
[0054] FIG. 3 shows a detail view of FIG. 2;
[0055] FIG. 4 shows light propagating in the optical PCB of FIG.
1;
[0056] FIG. 5 shows a schematic section through an example of an
optical PCB according to an embodiment of the present
invention;
[0057] FIG. 6 shows a detail view of FIG. 5;
[0058] FIG. 7 shows light propagating in the optical PCB of FIG.
5;
[0059] FIG. 8 shows an example of an optical PCB having an
out-of-plane facet in accordance with an embodiment of the present
invention;
[0060] FIG. 9 shows an orthographic view of the PCB of FIG. 5;
[0061] FIG. 10 shows a frame for use with the PCB of FIG. 5;
[0062] FIG. 11 shows an example of an optical PCB having a lens
array in accordance with the present invention;
[0063] FIG. 12 shows a cross section view of FIG. 11;
[0064] FIG. 13 shows an example of an optical PCB having a
connector slot in accordance with the present invention;
[0065] FIG. 14 shows a cross section view of FIG. 13;
[0066] FIG. 15 shows a cross section view of an example of an
optical PCB having a lens array and a connector slot in accordance
with the present invention;
[0067] FIGS. 16 to 19 show another example of a method of providing
a film to a waveguide facet in accordance with an embodiment of the
present invention;
[0068] FIGS. 20 and 21 show from the front and the rear an example
of a slide holder for use with an embodiment of the present
invention;
[0069] FIG. 22 shows the slide holder of FIGS. 20 and 21 applied to
an optical PCB and FIG. 23 shows a cross section of this
arrangement; and,
[0070] FIGS. 24 to 27 show an example of a method of providing a
film to a waveguide facet using the slide holder of FIGS. 20 and 21
in accordance with an embodiment of the present invention.
[0071] FIG. 5 shows a section through an example of an optical PCB
2 according to an embodiment of the present invention. As with the
optical PCB shown in FIG. 1, the optical PCB 2 of FIG. 5 comprises
a base or support layer 4 on which is arranged a lower cladding
layer 6. A plurality of optical waveguides 8 are provided in a
higher refractive index core layer on the lower cladding layer 6
(only one being visible in FIG. 5), and are surrounded by an upper
cladding layer 10.
[0072] The method of manufacture of the optical printed circuit
board 2 can be any suitable method. Suitable techniques for
manufacturing an optical PCB 2 are well known in the prior art, and
so the manufacture of the optical PCB 2 is not discussed in detail
herein. Nonetheless, a brief overview of a preferred method is now
given.
[0073] To manufacture an optical printed circuit board 2 as shown
in FIG. 5, initially, a support layer 4 composed of a common PCB
material such as FR4 or a flexible laminate material such as
polyimide is provided. On the support layer, a lower cladding layer
6 is provided. An optical core layer (from which the optical
waveguides 8 will be formed) is then formed on the lower cladding
layer 6. Using conventional lithographic techniques and/or other
conventional PCB manufacturing techniques, channels of the material
of the optical core layer which will eventually form the optical
waveguides are formed. These may be formed using any known
lithographic techniques. Typically, a layer of a thermally and/or
ultra-violet radiation curable liquid polymer such as a
polyacrylate or polysiloxane is deposited on the lower cladding
layer 6. This is spin coated to ensure uniform deposition of the
liquid polymer layer on the lower cladding layer, with the
thickness of the layer being controlled by a standard technique
such as doctor blading to ensure uniformity of thickness across the
layer. A mask is then arranged over the liquid polymer layer and
ultra-violet radiation is used to cure the exposed structure.
Alternatively some polymers can be thermally cured, in some cases
in combination with ultra-violet radiation. The mask is then
removed and the uncured (still liquid) material is removed. This
leaves a pattern of channels that will later form the optical
waveguides 8. The upper cladding layer 10 is then applied.
[0074] As well as lithographic techniques, other techniques can be
used in any or all of the steps of forming the optical PCB.
Examples include direct laser writing, direct electron beam
writing, laser ablation, embossing, ink jet printing or micro jet
printing.
[0075] The upper cladding layer 10 may be applied and either
uniformly cured to provide a uniform unstructured upper cladding
or, for some applications, structured using a second mask. For
example, a curable polymer may be provided in liquid form and then
a second mask may be used to selectively form a solid upper
cladding layer in desired regions. The tolerance required for the
formation of the upper cladding layer is typically significantly
lower than that required for the formation of the waveguides and
alignment features or projections. A more detailed description of a
suitable technique is disclosed in the co-owned US patent
application published as US-A-2009-0162004, the contents of which
are hereby incorporated by reference.
[0076] Clearly, many of the techniques used in the manufacture of
layered products such as the optical PCB shown in FIG. 2 are known.
A more detailed description will therefore not be provided.
[0077] A film 30 is applied over the end of the optical PCB 2. The
film 30 is a thin, adhesive-backed strip allowing it to be easily
stuck over the exposed planar waveguide interface 12. The film 30
has a refractive index matching that of the waveguide core 8 to
preferably within .+-.10% of the difference in refractive index
between the waveguide core and the free space medium (which is
typically air), and more preferably within .+-.1% of this
difference. Preferably the film 30 will be comprised of the same
material as the waveguide 8, which is especially useful if the
waveguide 8 is polymer.
[0078] As shown in FIG. 6, the soft, partially viscous texture of
the film 30 on the adhesive backed side 30a will effectively fill
in the rough profile 14 of the cut waveguides 8 and eliminate
scattering at this interface 12.
[0079] The outer surface 30b of the film 30 will preferably be over
an order of magnitude smoother than that of a typically polished or
laser ablated waveguide end facet 12 and will therefore
significantly reduce scattering loss in these areas, as shown by
FIG. 7.
[0080] Thus, this technique damps or eliminates scattering of
propagating light at the waveguide end facet 12. Although FIG. 4
shows scattering of light exiting the waveguides 8, the same
principle applies to light entering the waveguides 8. Similarly,
although FIGS. 6 and 7 show an in-plane facet 12, the invention is
applicable to out-of-plane facets 12 also. FIG. 8 shows an example
of an optical PCB 2 having an out-of-plane facet 12. As can be
seen, the waveguide 8 runs parallel to the plane of substrate until
near its end, at which point it extends out of plane and terminates
at the top surface of the upper cladding layer 10. An angled
portion 16 of the waveguide 12 reflects the light between the
in-plane and the out-of-plane portions of the waveguide 8. A film
30 is applied over waveguide facet 12.
[0081] Suitable adhesive-backed index matching film is for example
manufactured by Japanese company "Tomoegawa"
(http://www.tomoegawa.co.jp/english/).
[0082] As well as providing a low cost and improved processing for
finishing a waveguide 8, the solution is also less prone to damage
the waveguide 8 compared with the precarious techniques currently
used in the manufacture of optical PCBs 2, namely the correct
surface finish by polishing or such line of all in-plane or
out-of-plane waveguides 8. Thus the preferred embodiments of the
present invention improve yield when manufacturing optical PCBs
2.
[0083] Discrete films 30 can be applied to each of the various
waveguide facets 12. Alternatively, as shown in FIG. 9, a
continuous section of film 30 can be applied across a plurality of
waveguide ends having coplanar facets 12 in a localised region.
This improves efficiency of applying the films 30 to the facets 12
and reduces material and assembly cost.
[0084] Preferably the film 30 will be replaceable, e.g. by choosing
a film 30 with a suitable adhesive. This allows a film to be
removed and a new film 30 to be applied in its place. This can be
helpful in the situation where the previous film 30 is damaged or
worn. In the prior art, there is no practicable way of repairing a
damaged end of a waveguide 8.
[0085] Referring now to FIG. 10, in a preferred embodiment, a
holding frame 32 will be attached to the PCB 2 over the adhesive
film 30 to hold the film 30 in place. The frame 32 may attach to
the PCB 2 by any suitable means, e.g. clip, fastener, adhesive etc,
and contacts the film 30 around some or all of its periphery to
hold it in place. This helps guard against accidental removal of
the film 30 and/or damage to the film 30. The frame 32 may also
hold the film 30 taut in order to help prevent wrinkles or other
mechanical deformations developing in the film 30 when the PCB 2 is
deployed in an operational environment.
[0086] In typical optical PCB 2 applications a mechanical
receptacle is usually applied around the waveguides to accommodate
pluggable connectors. This receptacle may also incorporate a lens
array, which is held at a precise distance away from the waveguides
as part of a free space coupling scheme. Due to the fabrication
tolerances inherent to a PCB 2, it is difficult to ensure that this
precise distance is maintained.
[0087] The preferred holder 32 can also include a connector
receptacle and/or lens holder as part of a free-space coupling
arrangement. FIG. 11 shows a preferred holding frame 32 to which is
attached a lens plate 34 in which is formed an array of lenses 36.
The lenses 36 are aligned with the waveguide facets 12 so as to be
able to couple light into and/or out of the waveguides. FIG. 12
shows the holding frame 32 and lens array 36 in cross section. As
the waveguide film 30 has a defined and very smooth outer surface
30b, the holding frame 32 serves as a far better mechanical
registration point in the z-axis (along the axis of the waveguides
at the interface) to allow accurate coupling of light into and out
of the waveguides 8.
[0088] FIG. 13 shows an embodiment where the holding frame 32 has
connector slots 38 or other attachment means by which connectors
can be attached to the optical PCB 2. FIG. 14 shows this
arrangement in cross section. The connectors can be attached to
allow light to be coupled to and from the waveguides 8. Again,
using the holding frame 32 allows accurate alignment of the
connectors and the waveguide facets 12.
[0089] FIG. 15 shows a holding frame 32 which supports a lens plate
34 and has connector slots 38 to allow connectors to be attached to
the optical PCB 2 to accurately couple light to and from the lens
array 36 and thus to the waveguides 8.
[0090] FIGS. 16 to 19 show another example of a method for
providing a film on a waveguide facet 12 in accordance with an
embodiment of the present invention. First, as shown in FIG. 16, a
liquid UV curable index matching material 40 is applied to the
waveguide end facet 12. As shown in FIG. 17, this is then pressed
down into a thin film by a transparent plane 42 with a very smooth
surface e.g. a glass slide. The liquid material 40 flows into and
fills the troughs of the rough surface of the facet 12 (for example
the rough surface 14 shown in FIG. 3). As shown in FIG. 18, the
material 40 is then exposed to UV light through the transparent
plane 42 causing it to be cured. After curing, as shown in FIG. 19,
the transparent plane 42 is removed leaving a hard permanent film
30 with a surface roughness on the outer surface 30b similar to
that of the transparent plane 42. Accordingly a transparent plane
42 should be chosen with a surface roughness at least an order of
magnitude less than that of the waveguide facet 12.
[0091] Preferably a release agent is applied to the transparent
plane 42 before being applied to the liquid material 40 to prevent
the material 40 sticking to the transparent plane 42 during the
curing process, allowing the transparent plane 42 to be removed
after curing and leaving negligible distortion to the outer surface
of the cured film 30.
[0092] Preferably the index matching material 40 is the same
material as the waveguide core 8 itself. This is practical in cases
where the waveguide core 8 is a liquid UV curable polymer. In any
case, preferably the film 30 has an optical index which matches the
optical index of the waveguide 8 to within .+-.10% of the
difference between the waveguide core 8 refractive index and the
external environment, e.g. air.
[0093] This provides a film 30 on the facet 12 which has a similar
effect to the film 30 described in relation to FIGS. 5 to 7 in
reducing scattering loss at the waveguide facet 12 by providing a
smooth surface at the outer surface of the film 30. This technique
has been found to reduce scattering losses by 3 dB.
[0094] In some applications, the upper cladding layer 10 will also
be structured to create trenches around exposed waveguide core
sections allowing passive mechanical registration such as described
in co-pending patent US-A-2009-0162004. It is important that when
applying the index matching material 40 these trenches are left
completely uncontaminated. This may prove to be difficult as the
trenches are in very close proximity to the waveguide facets 12 so
potentially there is a risk from the uncured liquid from flowing
into them.
[0095] To address this, in a preferred embodiment, a removable
passive component is provided to fit into the trenches, filling
them and preventing the uncured liquid from flowing into them.
Preferably the passive component also holds the transparent plane
42 allowing convenient and repeatable application of this
technique. For example, FIGS. 20 and 21 show a suitable plane
holder 50 from the front and from the rear respectively. FIG. 22
shows the plane holder 50 and transparent plane 42 applied to an
optical PCB 2 and FIG. 23 shows this in cross section. As can be
seen, shaped projections 51 on the plane holder 50 fit into the
trenches 19 on the PCB 2, thereby aligning the holder 50 with the
PCB 2 and also shielding the trenches 19 from the uncured
liquid.
[0096] FIGS. 24 to 27 show a film 30 being provided on the
waveguide facet 12 using the same method as shown by FIGS. 16 to 19
with the plane holder 50 being used to shield the trenches 19.
[0097] Embodiments of the present invention have been described
with particular reference to the examples illustrated. However, it
will be appreciated that variations and modifications may be made
to the examples described within the scope of the present
invention.
* * * * *
References